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Date: Tue, 16 Feb 2016 15:43:06 +0100
From: Petr Mladek <pmladek@suse.com>
To: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Cc: Andrew Morton <akpm@linux-foundation.org>, Jan Kara <jack@suse.com>,
        Tejun Heo <tj@kernel.org>, Kyle McMartin <kyle@kernel.org>,
        Dave Jones <davej@codemonkey.org.uk>,
        Calvin Owens <calvinowens@fb.com>, linux-kernel@vger.kernel.org,
        Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>
Subject: Re: [PATCH v4 2/3] printk: set may_schedule for some of
 console_trylock callers
Message-ID: <20160216144306.GU12548@pathway.suse.cz>
References: <1455302232-2252-1-git-send-email-sergey.senozhatsky@gmail.com>
 <1455302232-2252-3-git-send-email-sergey.senozhatsky@gmail.com>
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On Sat 2016-02-13 03:37:11, Sergey Senozhatsky wrote:
> console_unlock() allows to cond_resched() if its caller has
> set `console_may_schedule' to 1, since
> 'commit 8d91f8b15361 ("printk: do cond_resched() between lines while
> outputting to consoles")'.
> 
> The rules are:
> -- console_lock() always sets `console_may_schedule' to 1
> -- console_trylock() always sets `console_may_schedule' to 0
> 
> However, console_trylock() callers (among them is printk()) do
> not always call printk() from atomic contexts, and some of them
> can cond_resched() in console_unlock(), so console_trylock()
> can set `console_may_schedule' to 1 for such processes.
> 
> For !CONFIG_PREEMPT_COUNT kernels, however, console_trylock()
> always sets `console_may_schedule' to 0.
> 
> It's possible to drop explicit preempt_disable()/preempt_enable()
> in vprintk_emit(), because console_unlock() and console_trylock()
> are now smart enough:
> a) console_unlock() does not cond_resched() when it's unsafe
>   (console_trylock() takes care of that)
> b) console_unlock() does can_use_console() check.
> 
> Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>

It looks safe after all.

Reviewed-by: Petr Mladek <pmladek@suse.com>

Best Regards,
Petr